Sorry, I ended up blending a couple discussions there.

The difference in the parameter is just so they are normalized, you already have the first part of the compound key, you don't need it again. So you don't have these weird corner-cases where a user can even try to make a message from MessageBoxA a reply to a message from MessageBoxB, like that content-association bug we found when comments were originally written. IMO structuring your methods such that corner cases like that exist tells me that they are denormalized, they don't properly reflect the structure of your API model, and it makes them more confusing.

I think I've said all I can about it. If you still want it changed just let me know and I'll go ahead with it.